It looks like this, outside and inside:
It seems fine as a meter, with one functional exception - the continuity tester is completely useless due to a delay on the detection/beep. At a guess (but no data to verify ...actually maybe that's in the datasheet. I should check) it's done in software and polled every 250ms or so.
As I write this, there's just 8 minutes left before the provided half-unwrapped battery expires!
Also the fuses are not up to spec (250v not 600v) so this is not safe for high voltage stuff.
It's based around the FS9922-DMM4 (datasheet linked at the end), with the USB interface provided by a nicely opto-isolated CP2102. I looked at the USB connector thinking "huh?" and wondering how on earth it actually worked, but for some reason (via symmetry?!) they bridged D+ and ID:
It spits data out at 2400bps (format spec in datasheet), and the included software will run in WINE if you must. Note that on a real Windows PC it does not connect with a high COM port number. I was given COM21 as default and changing that to COM2 it worked fine.
The FS9922 is used in a bunch of other meters so is already supported by sigrok. It currently needs adding to the list of supported meters but I'll submit a patch after posting this. E.g.
diff --git a/src/hardware/serial-dmm/api.c b/src/hardware/serial-dmm/api.c
index af70fc3..905b239 100644
@@ -560,4 +560,10 @@ SR_REGISTER_DEV_DRIVER_LIST(serial_dmm_drivers,
2400, DTM0660_PACKET_SIZE, 0, 0, NULL,
sr_dtm0660_packet_valid, sr_dtm0660_parse, NULL
+ "mercury-mttr01-ser", fs9922,
+ "Mercury", "MTTR01", "2400/8n1/rts=0/dtr=1",
+ 2400, FS9922_PACKET_SIZE, 0, 0, NULL,
+ sr_fs9922_packet_valid, sr_fs9922_parse, NULL